During Saturday morning, regions such as Camden and Liverpool had "fair" air quality readings, which carry the warning that "unusually sensitive people should consider reducing prolonged or heavy outdoor exertion", according to OEH.

A similar alert for poor air quality in the Sydney region has been issued for Sunday with small particle - PM2.5 - levels forecast to exceed national air quality standards.

More than 150 firefighters from the RFS, Fire & Rescue NSW and National Parks & Wildlife Service worked on western containment line of the Holsworthy fire on Saturday, with 70 crews scheduled to work overnight, an RFS spokesman said.

As of last Saturday afternoon, there were 34 bush and grass fires across NSW, with 14 uncontained.

“Strengthening winds are expected on Sunday which will be a concern for fires burning in NSW. In northern NSW,” the spokesman said. “The winds may put pressure on containment lines at the Mackel Airfield fire near Campbelltown.”

Very dry conditions across much of NSW - and the prospect of little relief in spring - last week prompted authorities to bring forward the start of the official fire season for part of the state's north-east.

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Inspector Shepherd said it was "more than likely" some regions that would normally start their formal fire season on October 1 will instead declare them a month earlier.

"We're heading [through] an extremely dry winter, with an extremely dry landscape, and that includes around Sydney," Inspector Shepherd said.

While the RFS does not formally release bushfire threat ratings at this time of year, internal monitoring indicates large parts of the state would have "high" fire danger levels this weekend, he said.

Dry with record warmth

A late shower for Sydney on Saturday may be the last for the month, which is currently shaping up to be Sydney's driest July since 1995. Rainfall is roughly a tenth of the 96.6 millimetres typical for July.

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On current forecasts, the city will likely set a record for July maximums, with temperatures running more than three degrees above the long-run norm of 16.4 degrees, according to the Bureau of Meteorology. Just one day this month has failed to meet the daytime average.

Sunday's top is predicted to reach 24 degrees.

Even with the clear skies bringing a chilly start to many mornings, minimum temperatures have also been slightly above the long-run average of 8.1 degrees.

Strengthening winds on Sunday should help dissipate the smoke from fires but they may also add to the risk if new fires are lit or existing ones escape controls.

"We want people to be extremely vigilant" when lighting fires, and to notify neighbours and the RFS, Inspector Shepherd said.

For the first six months of the year, Sydney's average maximum temperature was 24.6 degrees, or the second warmest on record behind only 2016, the bureau said.

Statewide, NSW had its hottest first half year on record, smashing the previous record - set just a year earlier - by more than half a degree.

Aircraft in demand

Inspector Shepherd said authorities had more than a 100 aircraft on call to help with fire-fighting. The bigger planes, though, would typically not arrive from North America and Europe until early September.

With large fires in Europe this summer - including large week’s tragic blazes in Greece – and additional deadly fires in California, demand for water-bombing aircraft is high.

“We are slowly edging into each other’s cross-over seasons,” Inspector Shepherd said, noting the fire seasons in both hemispheres are getting longer in a warming world.

The RFS deployed one helicopter to assist ground crews with mapping and intelligence.